1. Field of the Invention
The invention is relates to the removal of scale from metal surfaces, and more particularly, to the removal of scales containing fluorides from metal surfaces.
2. Description of the Prior Art
When coal or other ash-containing organic materials are gasified in a high-pressure, high-temperature partial oxidation quench gasification system, the ash material commonly becomes partitioned between coarse slag, finely divided slag particles, and water-soluble ash components. Water is used in the system to slurry the feed coal, to quench the hot synthesis gas, also referred to as "syngas" and to quench the hot slag byproduct. Water is also used to scrub particulate matter from the syngas, and to assist in conveying the slag byproduct out of the gasifier.
Calcium fluoride and magnesium fluoride scale which forms on evaporator tubes is usually chemically removed by inorganic acids such as sulfuric, hydrochloric, or nitric acids. When sulfuric acid is used for scale removal, CaSO.sub.4 is sometimes precipitated. During acid cleaning of fluoride scale, corrosive hydrofluoric acid is formed in the cleaning solution and certain metals and metal alloys, such as titanium, nickel, and stainless steel can become subject to severe corrosion from the hydrofluoric acid. The presence of fluoride ion (F.sup.-) in the solution interferes with the protective oxide films that form on these metals and allows for dissolution of the titanium, iron, and nickel ions in an acidic solution. Therefore, chemical cleaning of fluoride scale by the use of acids alone in process equipment is not practical. It is also noted that calcium scale can be chemically removed by use of ethylene diamine tetracetic acid.
Scale can also be removed by mechanical means such as by scraping or by impact with a hammer or by hydroblasting. However, chemical cleaning is preferred and is usually more thorough because scale can be dissolved and removed in places where a hydroblasting nozzle cannot reach. It is therefore desirable to chemically dissolve fluoride scale from equipment constructed of titanium or stainless steel. Titanium and stainless steels are commonly used in the wastewater treatment industry, especially in the construction of wastewater evaporators.
The literature has also addressed the problem of hydrofluoric acid corrosion in process equipment made of stainless steels, nickel alloys and titanium alloys. Koch, G. H., "Localized Corrosion in Halides Other Than Chlorides," Environment Effects, June 1993 discloses that ferric or aluminum ions can inhibit corrosion.
The effect of water solutions and their corrosiveness in flue gas desulfurization process scrubbers has also been studied. These solutions contain chlorides, fluorides and sulfates at low pH, for example, 4800 mg/kg fluoride at a pH of 1. The addition of flyash minerals which contain significant amounts of silicon, iron, and aluminum can inhibit corrosion of titanium in otherwise aggressive fluoride containing solutions. It was also found that if 10,000 mg aluminum/kg (added as aluminum sulfate) were added to a corrosive acidic solution containing 10,000 mg/kg chloride and 1,000 mg/kg fluoride, the solution is no longer corrosive to titanium.